1. Field of the Invention
The present invention relates to a method for grinding a bar in which thin film magnetic elements with inductive electromagnetic transducers and MR sensors are formed.
2. Description of the Related Art
A hard disk drive generally uses a thin film magnetic head. Among other kind of heads, a composite thin film magnetic head has been widely used, which has a stacked structure comprising a read head having a magnetoresistive sensor (also referred to as MR sensor) for reading magnetic data and a write head having an inductive electromagnetic transducer for writing magnetic data.
A thin film magnetic head of this kind is fabricated according to the following steps. First, a plurality of thin film elements each having an MR sensor and/or an inductive electromagnetic transducer in a stacked structure are formed on a ceramic substrate. Then the wafer on which the thin film elements are formed is cut into elongate bars such that a plurality of thin film elements are aligned in a single line. The cut bars are then ground by a special grinding machine to form an air-bearing surface.
FIGS. 1A to 1C exemplarily show a conventional method for grinding a bar in which thin film elements are formed in line. A wafer on which thin film magnetic elements are formed in two dimensions is cut line by line into bars 172. Bar 172 is carried on holder 156 as shown in FIG. 1A, and pressed against a rotating grinding disc 152 of a grinding machine while being carried on holder 156 as shown in FIG. 1B, whereby the entire grind surface of bar 172 is ground by grinding disc 152.
However, if bar 172 is pressed against grinding disc 152 in a tilted position, bar 172 is ground unevenly in the longitudinal direction of bar 172, i.e., in the direction in which the thin film magnetic elements are aligned. In other words, the inner circumferential portion is ground excessively while the outer circumferential portion is ground insufficiently as shown in FIG. 1C. In FIG. 1C, when bar 172 is ground to meet the line of “target MRH” shown in the figure, thin film magnetic element 161a disposed in radially outward portion will not be ground to the target MRH, thin film magnetic head 161b in the center portion will be ground to the target MRH, and thin film magnetic head 161c disposed in radially inward portion will be ground exceeding the target MRH.
For reference, the term “Air bearing surface (ABS)” is used herein to mean a surface of a thin film magnetic head facing a recording medium. The term “MR height (MRH)” is used to mean a height or length of an MR sensor measured perpendicularly from an ABS to the opposite end.”
As an MR sensor, for example, an AMR (Anisotropic magneto Resistive) sensor utilizing an anisotropic magnetoresistive effect, a GMR (Giant Magneto Resistive) sensor utilizing a giant magnetoresistive effect, or a TMR (Tunnel Magneto Resistive) sensor utilizing a tunnel magnetoresistive effect are commonly used. It is important to form an MRH according to design requirement in order to ensure the output performance. For this reason, in the prior art, a wafer is formed with an extra amount of MR height. When the wafer is cut into bars, this surplus portion still remains. Then the bar is ground to ABS, and the surplus portion of the MR sensor is removed to form the target MRH.
Thus, if the amount that is ground varies depending on the position within the bar as described above, MRH varies from one thin film magnetic head to another, entailing significant degradation of the yield ratio. In order to address such an issue, a method has been disclosed in which resistive films, which are dummy films, are embedded in the grind surface of the bar in advance. Since the resistive films vary in electric resistance as the resistive films are ground, the amount that is ground can be controlled by monitoring the resistance value of the resistive films. For example, see specifications etc. of Japanese Patent Laid-down Publication No. 101634/2001 and 242802/99.
However, in the above prior art, while the amount that is ground are properly controlled in the longitudinal direction of the bar, the amount that is ground cannot be properly controlled in the direction perpendicular to the longitudinal direction of the bar, i.e., in the short direction of the bar, which will be explained below in detail.
A thin film magnetic head typically has an inductive electromagnetic transducer on the upper side of an MR sensor in the stacked direction. The inductive electromagnetic transducer has an upper magnetic pole layer and a lower magnetic pole layer, and pole tip regions of the both layers are opposite to ABS with a write gap sandwiched therebetween. For reference, the term “throat height (TH)” is used herein to mean the length (height) of a region in which both magnetic pole layers are opposite to each other with the write gap interposed, i.e., the length of pole tip regions from ABS to the opposite end. Since throat height TH is a key dimension for writing characteristics, it is desirable to form throat height TH accurately according to design requirement.
The drawing on the right in FIG. 1C schematically shows a distribution of the amount that is ground in the short direction of bar 172. MR sensor 161m and write head portion 161t are arranged side by side in the short direction. As has already been described, the amount that is ground of bar 172 in the longitudinal direction is controlled by using MRH as the target, so that thin film magnetic element 161b in the center portion shown in the figure is ground substantially to meet the target MRH. However, in reality, bar 172 is subject to a pitch motion in the circumferential direction in the grinding process due to various causes such as deformation of holder 156 and bar 172 themselves, the rigid adhesion of bar 172 to holder 156, and the variation in the position of bar 172 when attached to holder 156. This results in variation in the amount that is ground in the forward and backward direction of bar 172, i.e., in the rotational direction of grinding disc 152. While as shown in the drawing on the right in FIG. 1C, a larger amount of bar 172 may be ground in the forward portion as described above the arrow in the figure, the bar 172 may in other cases be ground in a larger amount in the backward portion as described below the arrow in the figure. The direction of the pitch motion is unsteady. In the actual grinding process, the amount that is ground may vary in both the longitudinal and perpendicular directions. However, the amount that is ground can be properly controlled in the longitudinal direction by the above conventional method.
As mentioned above, the amount that is ground was controlled by taking MRH as a reference in the prior art, because MRH is much smaller than throat height TH, and hence MRH is more likely to be subjected to greater influence by the variation in grinding. In fact, it was possible in the prior art to suppress the variation of throat height TH to a small extent by controlling MRH. It will be required, however, to improve accuracy in the dimension of throat height TH, as well as to suppress, as a matter of course, the variation of MRH, in the current tendency toward even higher areal density.
It has become difficult to improve the processing accuracy of throat height TH by conventional grinding methods, in which the processing accuracy of throat height TH depends on that of MRH, and there exists a need to control the processing accuracy of throat height TH itself.